feat: refactor user registration steps into modular functions and implement step registry

- Replaced individual step imports with a centralized step registry in background.js
- Created separate modules for each registration step: open-chatgpt, submit-signup-email, fill-password, fetch-signup-code, fill-profile, oauth-login, fetch-login-code, confirm-oauth, platform-verify
- Updated tests to reflect changes in step imports and added new tests for step registry functionality

tests/background-step-registry.test.js

@@ -0,0 +1,11 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+
+test('background imports step registry module and uses sparse order values', () => {
+  const source = fs.readFileSync('background.js', 'utf8');
+  assert.match(source, /background\/steps\/registry\.js/);
+  assert.match(source, /order:\s*10/);
+  assert.match(source, /order:\s*90/);
+  assert.match(source, /stepRegistry\.executeStep\(step,\s*state\)/);
+});